1. Field of the Invention
The present invention relates to a Wavelength Division Multiplexing (WDM) submarine optical repeater system and particularly to a submarine optical gain equalizer installed on a submarine optical transmission line, and the submarine optical transmission line and its installation method.
2. Related Background Art
In the WDM optical transmission system, to transmit multi-wavelength optical signals in a long distance, it is necessary to install light amplifiers at predetermined intervals on an optical transmission line so as to amplify signal light. The light amplifiers, however, have gain characteristics slightly different among wavelengths, which causes level differences between optical signals having longer wavelengths and shorter wavelengths. These level differences are accumulated in each light amplifier. Accordingly, some lines are disabled from being used for communications in long-distance optical transmission lines. Therefore, in a short optical transmission system over a transmission distance of several hundreds of kilometers or so, light source power of signal light having different wavelengths has been previously adjusted based on the characteristics of the light amplifiers, for example.
In a submarine optical communication system, however, with multi-wavelength light having an optical transmission line exceeding 10,000 km, for example, a method of adjusting the light source power of the signal light does not function appropriately in some cases. In the submarine optical communication system including such a long-distance optical transmission line, there has been no proposal of any specific configuration of gain equalization for adjusting respective wavelength levels. It is also possible theoretically to achieve an equipment for detecting respective wavelength levels automatically and for adjusting the levels. It is presumed, however, that this kind of the equipment is extremely complicated and requires high cost.